The invention relates to a process for selective production of polymerization-quality propylene from an olefinic C4 fraction.
The steam-cracking of feedstocks that consist of light paraffinic fractions produces the ethylene and the propylene that are necessary to petrochemistry. It also produces a certain number of other heavier products, and in particular a C4 hydrocarbon fraction that contains mainly butadiene-1,3, isobutene, n-butenes and butanes, accompanied by traces of acetylenic hydrocarbons.
The catalytic cracking of heavy hydrocarbon feedstocks produces, alongside gasoline and gasoil fractions that are the main products, lighter products, including a C4 hydrocarbon fraction that contains mainly isobutane, isobutene, n-butenes and butanes, accompanied by small amounts of butadiene-1,3 and acetylenic hydrocarbons.
Until recently, only butadiene-1,3 and isobutene were used in the polymer industry, in particular in the tire industry. The increase of the longevity of tires and a relative stagnation of the demand ensure that there is now excess butadiene that is not used or is poorly used. To date, isobutene was used, for example, for the synthesis of ethers with the use of additives in automobile fuels or as a monomer in the synthesis of polyisobutene. These uses, however, can lead to saturation and render the isobutene useless.
This invention proposes a process for treatment of a C4 hydrocarbon fraction that contains primarily isobutene, n-butenes, butanes, and butadiene-1,3 in a variable amount that includes the skeletal isomerization of isobutene into n-butenes and that makes it possible to transform all of the C4 unsaturated compounds into propylene that can be used for, for example, polymerization.
The fractions that are treated in the process according to the invention correspond to the C4 fractions of conversion processes. They can correspond to, for example, the crude C4 fraction for steam-cracking, the C4 fraction for steam-cracking after extraction of the butadiene that is commonly called raffinate-1, or the C4 fraction for catalytic cracking.
The relative proportions of ethylene and propylene that are produced in a steam-cracking operation can be modulated to a certain extent by changing the nature of the feedstock and by modifying the operating conditions (the degree of rigor) of the cracking. The operating method that is oriented toward a larger proportion of propylene, however, inevitably entails a decline in the yield of ethylene and a higher C4 fraction and gasoline fraction production.
Another object of this invention is thus to increase the propylene production while maintaining a high ethylene yield with the treatment of the C4 hydrocarbon fraction and therefore without it being necessary to reduce the rigorous conditions of the steam-cracking device.
The process that is the object of the invention is more specifically a process for converting into propylene an olefinic C4 fraction, whereby said fraction comprises diolefins, primarily butadiene-1,3, butene-1, butene-2, isobutene and acetylenic impurities, and whereby said process comprises the following stages that take place successively:
1) the selective hydrogenation of diolefins and acetylenic impurities with isomerization of butene-1 into butenes-2, carried out in a reactor, in the presence of a catalyst, in order to obtain an effluent that contains for the most part butenes-2 and isobutene, and that contains virtually no diolefins or acetylenic compounds;
2) the separation by distillation of a top fraction that contains for the most part isobutene and unconverted butene-1 in the first stage, and a bottom fraction that contains essentially butene-2 and butane; and
4) the metathesis of the butenes-2 fraction that is obtained from stage 2 with the ethylene so as to obtain an effluent that contains propylene, whereby the metathesis is followed by a separation of the propylene;
whereby said process also comprises a stage 3 of skeletal isomerization of the isobutene into n-butenes in the top fraction, with recycling of at least a portion of the effluent in stage 1.
The isomerization of butene-1 into butenes-2 as carried out in stage 1 can also be carried out in part in association with the distillation (stage 2) by using an isomerization catalyst as described for stage 1 according to the teachings of French FR-B-2 755 130, in the name of the applicant.
The special conditions of the different stages of the process according to the invention, carried out from a C4 hydrocarbon fraction that contains primarily isobutene, n-butenes, butanes, as well as butadiene in a variable amount, whereby said C4 fraction is subjected to these stages to produce essentially propylene, are described in more detail below.
The main object of the first stage is to transform the butadiene and the n-butenes into butenes-2. Actually, the butenes-2 are the source of the propylene that is produced in stage 4 of metathesis in the presence of ethylene. It is therefore desirable to maximize the butenes-2 yield, i.e., to draw as close as possible to the ratio that is allowed by thermodynamics. The second object of this stage is to eliminate the acetylenic hydrocarbon traces that are always present in these fractions and that are poisons or contaminants for the subsequent stages.
In this first stage, the following reactions are thus carried out simultaneously in the presence of hydrogen:
the selective hydrogenation of butadiene into a mixture of n-butenes;
the isomerization of butene-1 into butenes-2 to obtain a distribution that is close to the thermodynamic equilibrium; and
the selective hydrogenation of the acetylenic hydrocarbon traces into butenes.
These reactions can be carried out with various specific catalysts that comprise one or more metals, for example from group 10 of the periodic table (Ni, Pd or Pt), deposited on a substrate. A catalyst that comprises at least one palladium compound that is fixed on a refractory mineral substrate, for example on an alumina, is preferably used. The palladium content in the substrate can be 0.01 to 5% by weight, preferably 0.05 to 1% by weight. Various pretreatment methods that are known to one skilled in the art optionally can be applied to these catalysts to improve the selectivity in the hydrogenation of butadiene into butenes at the expense of the total hydrogenation of butane that it is necessary to avoid. The catalyst preferably contains 0.05 to 10% by weight of sulfur. Advantageously, a catalyst is used that comprises palladium that is deposited on alumina, and sulfur.
The catalyst can advantageously be used according to the process that is described in Patent FR-B-2 708 596. According to this process, the catalyst is treated, before it is loaded into the hydrogenation reactor, by at least one sulfur-containing compound that is diluted in a solvent, then the catalyst that is obtained that contains 0.05 to 10% by weight of sulfur is loaded into the reactor and activated under a neutral atmosphere or a reducing atmosphere at a temperature of 20 to 300xc2x0 C., a pressure of 0.1 to 5 MPa and a VVH of 50 to 600 hxe2x88x921, and the feedstock is brought into contact with said activated catalyst.
The use of the catalyst, preferably with palladium, is not critical, but it is generally preferred to use at least one down-flow reactor through a catalyst fixed bed. When the proportion of butadiene in the fraction is large, which is the case, for example, of a steam-cracking fraction when it is not desired to extract the butadiene from it for specific uses, it may be advantageous to carry out the transformation in two reactors in series to better monitor the selectivity of the hydrogenation. The second reactor can have a rising flow and play a finishing role.
In some cases, it may be advisable to dilute the feedstock that is to be treated by said C4 fraction in which the butadiene is partially or totally hydrogenated.
The amount of hydrogen that is necessary for all of the reactions that are carried out in this stage is adjusted based on the composition of the fraction advantageously to have only a slight hydrogen excess relative to the stoichiometry.
The operating conditions are selected such that the reagents and the products are in liquid phase and such that they promote the formation of butenes-2. It may be advantageous, however, to select an operating mode such that the products are partially evaporated at the outlet of the reactor, which facilitates the thermal monitoring of the reaction. The temperature may vary from 0 to 200xc2x0 C., preferably from 0 to 150xc2x0 C. or better from 0 to 70xc2x0 C. The pressure may be adjusted to a value of 0.1 to 5 MPa, preferably 0.5 to 4 MPa and advantageously from 0.5 to 3 MPa, such that the reagents, at least in part, are in liquid phase. The volumetric flow rate may be from 0.5 to 20 hxe2x88x921 and preferably from 1 to 10 hxe2x88x921, with an H2/diolefin molar ratio of 0.5 to 5 and preferably 1 to 3.
The hydroisomerization reactor or reactors may advantageously be followed by a stabilization column that eliminates the traces of gaseous hydrocarbons that are optionally present in the feedstock hydrogen.
The object of the second stage is to separate by distillation the C4 fraction that is obtained from the preceding stage to obtain, on the one hand, a fraction that contains isobutene, isobutane and the majority of butene-1, on the other hand, a fraction that contains a small amount of butene-1, butenes-2 and n-butane. The isobutene that is thus concentrated is conducted to stage 3 of skeletal isomerization. The butenes-2 fraction is directed toward the metathesis stage.
To reduce as much as possible the butene-1 concentration in the effluent of the column head, it is possible to use a reactive distillation column that comprises, on the inside of the column or outside, one or more feedstocks of the catalyst that is used as described for stage 1. The reactive distillation column that is used can then be of any type. In a preferred arrangement, at least one zone that contains the catalyst is arranged. The mechanical arrangement of the catalyst in the catalytic zone or zones should be such that it disturbs the flows of vapor and liquid as little as possible between the two separation zones that frame it. The catalyst can be placed, for example, in a thin layer on perforated plates or on grids, or in bags that are suspended or laid on substrates that ensure their mechanical behavior, or any other way that is known to one skilled in the art. On the other hand, the catalyst can be placed in the column so that only an upward flow of liquid phase passes through it. It can also be arranged in the form of catalytic packing according to the different implementations that are known. The separation zones that frame the catalytic zones can comprise plates or packing.
One of the uses of the column can correspond to, for example, the one that is described in French Patent FR-B-2 755 130 in the name of the applicant.
The distillation top fraction that is rich in isobutene is subjected in stage 3 to a skeletal isomerization that is intended to transform the isobutene into n-butenes, which can be sent to the inlet of zone 1. The optionally present isobutene may be purged.
This skeletal-isomerization reaction can be carried out with catalysts that have an alumina base or more particularly activated or vapor-treated aluminas (U.S. Pat. No. 3,558,733) or that comprise compounds such as those of titanium (U.S. Pat. No. 5,321,195 of the applicant) and/or boron (U.S. Pat. No. 5,659,104 of the applicant) in the case of eta- or gamma-aluminas, halogenated aluminas (U.S. Pat. No. 2,417,647) or bauxite. Zeolites or molecular sieves that have a mono-dimensional microporous network (Patent Documents EP-A-523 838, EP-A-501 577 and EP-A-740 957 of the applicant) can also constitute active phases of skeletal-isomerization catalysts. The alumina-based catalysts are generally used in the presence of water at temperatures of from 200xc2x0 C. to 700xc2x0 C., at a pressure of 0.1 to 2 MPa, at a volumetric flow rate of 0.1 to 20 hxe2x88x921 and with a molar ratio of injected water to hydrocarbon of 0.1 to 10. The zeolitic catalysts are used without water, at a temperature of 200xc2x0 C. to 500xc2x0 C., under a pressure of 0.1 to 2 MPa and at a volumetric flow rate of 0.1 to 20 hxe2x88x921.
The skeletal isomerization of the isobutene into n-butenes is carried out preferably with a catalyst that comprises alumina and titanium at a temperature of 300xc2x0 C. to 570xc2x0 C., a pressure of 0.1 to 1 MPa, at a volumetric flow rate of 0.1 to 10 hxe2x88x921, and in the presence of water injection, whereby the molar ratio of injected water/olefinic hydrocarbons is 0.1 to 10.
A catalyst that contains alumina and 0.03 to 0.6% by weight of titanium and that can also contain 0.05 to 5% by weight of an oxide of an element of group IIIA, whereby this element advantageously is boron, will preferably be used in the invention. Before being brought into contact with the hydrocarbons of the feedstock, this catalyst advantageously will have undergone a water vapor treatment at a temperature of 120-700xc2x0 C. under a partial water vapor pressure that is greater than 0.05 MPa, for a period of 0.5 to 120 hours.
The bottom fraction of the distillation zone, rich in butenes-2, preferably contains at most 1% by weight of butene-1, advantageously at most 0.5% by weight, and at most 1% by weight of isobutene. The butenes-2 fraction that is obtained from stage 2 does not contain outside contaminants and can therefore be sent directly into the fourth stage of the process. In this last stage, the butenes-2 are reacted with ethylene to produce propylene by metathesis. Because of the small amount of butene-1 in the feedstock, the by-product formation is very limited.
The metathesis reaction of the ethylene with the butenes-2 can be catalyzed by varied metallic oxides that are deposited on substrates, for example, by molybdenum, tungsten or rhenium oxides. A catalyst that comprises at least one rhenium oxide that is deposited on a substrate that comprises a refractory oxide that itself contains at least alumina and that has an acidic nature, such as, for example, alumina itself, silica-aluminas or zeolites, is preferably used.
It is possible to cite, by way of preferred examples, the catalysts that comprise rhenium heptoxide that is deposited on a gamma-alumina, such as those described in U.S. Pat. No. 4,795,734. The rhenium content (expressed in metallic rhenium) can be 0.01 to 20%, preferably 1 to 15% by weight. The catalysts are subjected to, for example, a final thermal activation at a temperature of 400 to 1000xc2x0 C. for a period of 10 minutes to 5 hours under a non-reducing atmosphere.
The catalysts that comprise rhenium heptoxide that is deposited on an alumina can also be modified by the addition of an oxide of another metal. Such modified catalysts comprise, for example, rhenium in the oxide state, at a rate of 0.01 to 20% by weight expressed in metallic rhenium, deposited on a substrate that contains at least 75% by weight of alumina and 0.01 to 30% by weight of at least one oxide of a metal that is selected from the group that is formed by niobium and tantalum, as described in Patent FR-B-2 709 125. Another class of modified catalysts comprises rhenium in the oxide state, at a rate of 0.01 to 20% by weight expressed in metallic rhenium, deposited on a substrate that contains at least 75% by weight of alumina and 0.01 to 10% by weight of aluminum of a compound of formula (RO)qAIRxe2x80x2r, where R is a hydrocarbyl radical of 1 to 40 carbon atoms, Rxe2x80x2 is an alkyl radical of 1 to 20 carbon atoms, and q and r are equal to 1 or 2, with q+r equal to 3 (see Patent FR-B-2 740 056).
The metathesis reaction is carried out preferably in a liquid phase, without oxygen, oxidized compounds and moisture, and at a temperature of 0 to 200xc2x0 C., preferably 20 to 150xc2x0 C., under a pressure at least equal to the vapor pressure of the reaction mixture at the reaction temperature.
The catalyst can be used in a fixed bed. Since it must be regenerated frequently, however, it is then necessary to use at least two reactors in parallel, whereby one is in use while the other is being regenerated. A catalyst moving bed system as described in French Patent FR-B-2 608 595 is preferably used. The catalyst is drawn off at regular time intervals from the bottom of the reactor and transferred to a continuous regeneration system, from where it is sent to the top of the reactor.
Taking into account the limitations that are imposed by thermodynamics, the unconverted ethylene is fractionated in a first distillation column and recycled in the metathesis reactor. A second distillation column separates the propylene and the unconverted C4 hydrocarbons that can be recycled in the metathesis reactor or in another location of the process.
When the process is applied to a steam-cracking C4 fraction, it may be advantageous to integrate the metathesis unit with the cracking device to take advantage of the fractionation train of the latter. The ethylene that is obtained from the steam-cracking operation is then used in the metathesis stage.
The succession of treatments adopted in the process according to the invention has many advantages. The most reactive compounds of the fraction, in particular the butadiene-1,3 that is present in variable amounts, as well as the traces of acetylenic hydrocarbons, are transformed from the first stage and therefore will not be the cause of parasitic reactions in the following stages. Furthermore, the selective hydrogenation of diolefins (butadiene-1,3 and, if necessary, butadiene-1,2) into butenes, the hydroisomerization of butene-1 and the skeletal isomerization of isobutene into n-butenes make it possible to increase considerably the butenes-2 concentration in the fraction, which thereby promotes a high yield of propylene in the metathesis stage.
The fractionation of the fraction that is obtained from the hydroisomerization into isobutene and butene-1, on the one hand, and into butenes-2, on the other hand, makes it possible to concentrate the isobutene for the skeletal-isomerization stage, as well as the butenes-2 that are then subjected to metathesis.
In addition, in the following metathesis stage (stage 4), the low butene-1 content in the butenes-2-rich fraction makes it possible to obtain a propylene selectivity that is close to 100%. Actually, it is known that the butene-1 reacts with the butenes-2 to produce propylene and pentenes, and that it reacts with itself to produce hexenes. Pentenes and hexenes are by-products of low value, which it is necessary to eliminate, in a costly manner. The process therefore makes possible an appreciable increase of the propylene yield and facilitates the recycling of butenes-2 in the metathesis reactor, since there are few pentenes and hexenes to eliminate.